Squeegees

The squeegee. Its name inspires ridicule, while its simplicity makes it one of the most overlooked tools in screen printing. But when you consider the squeegee's role in the printing process and the range of attributes that characterize it, you quickly discover that this tool is anything but basic.

How do squeegees influence print quality? What steps can you take to optimize their performance? To find out, you need to understand the squeegee's primary functions during the printing process. (See "Squeegee Terminology" below.)

The purpose of the squeegee

Squeegees "blades" are typically made of polyurethane or other highly flexible and resilient plastics, known as elastomers. They are typically cast in liquid form, either in horizontal "troughs" or in a centrifuge, then cut and milled to provide smooth, sharp edges. The blade material is typically rolled for shipping and later cut to the specific length required for a particular application or press.

The squeegee has four functions that influence or are influenced by any number of processing variables:

1. Forcing the ink into the mesh--influenced by mesh opening size and ink viscosity

2. Keeping the mesh in contact with the substrate--influenced by screen tension and off-contact distance

3. Adapting the mesh to the surface of the substrate--influenced by substrate hardness, roughness, and evenness

4. Removing excess ink from the screen--influences thickness of subsequent printed ink layer, as well as image definition

Squeegee selection criteria

Squeegees can influence printing results, but only if other variables in the process are controlled. This means understanding the rheological properties of your inks, as well as using high screen tension, low off-contact distance, and press settings that don't oversaturate the screen with ink. After optimizing these variables for a particular job, it's time to select the squeegee based on three parameters: durometer, blade profile, and blade edge profile.

Durometer: Durometer refers to the "hardness" of the squeegee. It affects the squeegee's adaptability to the surface and determines the amount of printing force transmitted to the screen and the substrate. Soft squeegees are more adaptable and transmit less force than hard ones.

Blade profile: Profile is the shape of the squeegee's printing edge (Figure 1). This shape also determines the squeegee's adaptability. In a sense, profile fine tunes this adaptability and limits the amount of force transmitted to the printing surface.

The profile affects the relationship between the set squeegee angle and the effective squeegee angle relative to the screen in the direction of the print stroke

(Figure 2). The effective angle is the actual angle during printing, when the force of the print stroke deflects the squeegee beyond the set angle. The smaller the effective squeegee angle, the more ink that is forced into the screen ahead of the printing blade and the less ink that is scraped off the screen surface.

Depending on the profile you select, the squeegee will force more or less ink into the screen, thereby influencing the printed ink-deposit thickness. Profile options include the following:

Square/rectangular: This profile provides medium adaptability and maximum force. When using a square/rectangular profile, the effective angle approximates the set angle. This is the most frequently used profile in screen printing.

Beveled Edges: can be beveled in various ways to provide maximum adaptability and minimum force. With beveled profiles, the effective angle is always less than the set angle. Beveling is most effective when used in applications requiring squeegees less than 14 in. (36 cm) long.

Round with a round profile provide minimum adaptability and minimum force. With such squeegees, the effective angle is always very small and nearly independent of the set angle.

Blade edge profile: Not to be confused with blade profile, the squeegee's blade edge profile refers to its sharpness (Figure 3). Edge profile establishes the amount of ink that will ultimately be delivered to the substrate. The sharper the edge, the less ink it will deliver. Since the amount of ink going through the screen also affects the resolution of the image, the less ink that is deposited, the sharper the printed image.

Mixing these three parameters in different combinations presents a wide range of possible squeegee types. For practical use, however, the primary combinations can be grouped to provide 12 general squeegee types. Since a squeegee essentially can be either sharp or unsharp, and the profile can be basically one of three shapes, this gives rise to four possible combinations:

1. Square/rectangle profiles with sharp edge

2. Square/rectangle profiles with round edge

3. Beveled profiles with sharp edge

4. Round profiles with no edge

Each of these combinations can occur in one of three general durometers (soft, medium, and hard), for a total of 12 squeegee types.

Selecting the appropriate squeegee out of these 12 types depends on the nature of the product being printed and a near infinite variety of production variables, including the substrate, ink, image, equipment, and operator skills. Substrates, for example, can be hard or soft, smooth or rough, even or uneven, absorbent or nonabsorbent, etc. Inks may be opaque or transparent, tacky or nontacky, high or low viscosity, etc. The image can comprise high- or low-resolution detail, or it may simply be a coating that covers the entire surface of the substrate. The printing equipment you use may be new or old, well maintained or poorly treated, and properly or marginally set up. Operators may be accomplished craftsmen or newly trained employees without any experience. And, of course, you face a wide range of intermediate values between any two of these extremes.

All these situations add up to a vast number of possible product/processing combinations. For this reason, it is impossible to recommend any one of the 12 squeegee types without analyzing the appropriate processing variables. On the other hand, it is possible to correlate the 12 squeegee types with the general functions of the squeegee as shown in Table I.

Table 1

Squeegee profile/edge

Force, mesh opening, viscosity

Substrate contact, off-contact, screen tension

Adaptation to substrate surface

Printed ink-layer thickness, image definition

Soft Durometer

square/rectangular - sharp

medium force; use with large mesh openings and low-viscosity inks

use with low-tension screens or screens with minimum off-contact distance

use on rough and uneven surfaces, dented press beds, and poorly set up presses

mediocre thickness control and image definition

square/rectangular - round

medium force; use with large mesh openings and low-viscosity inks

use with low-tension screens or screens with minimum off-contact distance

use on rough and uneven surfaces, dented press beds, and poorly set up presses

poor thickness control and image definition; use for coating applications

beveled - sharp

medium force; use with large mesh openings and low-viscosity inks

use with low-tension screens or screens with minimum off-contact distance

use on uneven, contoured surfaces (containers, etc.)

mediocre thickness control and image definition

round - no edge

medium force; use with large mesh openings and low-to medium-viscosity inks

use with low-tension screens or screens with minimum off-contact distance

use on rough and uneven surfaces, soft absorbent materials, and worn presses

poor thickness control and image definition; use for coating applications

Medium Durometer

square/rectangular - sharp

high printing force; useful for a wide range of meshes and ink viscosities

use with high- or low-tension screens and with appropriate off-contact distance

moderate adaptation; use on even surfaces with minimum texture

good thickness control and image definition; use for coating applications

square/rectangular - round

high printing force; useful for a wide range of meshes and ink viscosities

use with high- or low-tension screens and with appropriate off-contact distance

poor adaptation; use on even surfaces with minimum texture

mediocre thickness control and image definition; use for coating applications

beveled - sharp

medium printing force; useful for a wide range of meshes and ink viscosities

use with high- or low-tension screens and with appropriate off-contact distance

excellent adaptation to a wide variety of uneven surfaces

good thickness control and image definition on equip that is set up properly

round - no edge

high printing force; useful for a wide range of meshes and ink viscosities

use with high- or low-tension screens and with appropriate off-contact distance

poor adaptation; use on even surfaces with minimum texture

mediocre thickness control and image definition; use for coating and low-res apps.

Hard Durometer

square/rectangular - sharp

highest printing force available; use on any mesh with high-viscosity inks

use on high-tension screens with low off-contact distance

very poor adaptation; use only with smooth, even materials on properly set up presses

excellent thickness control and definition on smooth, even substrates

square/rectangular - round

highest printing force available; use on any mesh with high-viscosity inks

use on high-tension screens with low off-contact distance

very poor adaptation; use only with smooth, even materials on properly set up presses

good thickness control and definition on smooth, even substrates

beveled - sharp

highest printing force available; use on any mesh with high-viscosity inks

use on high-tension screens with low off-contact distance

good adaptation; useful for most uneven surfaces

excellent thickness control and definition on most surfaces

round - no edge

highest printing force available; use on any mesh with high-viscosity inks

use on high-tension screens with low off-contact distance

very poor adaptation; use only with smooth, even materials on properly set up presses

good thickness control and definition on smooth, even substrates

This table can be used to broadly define the most important processing variables and the corresponding squeegee types for 75% of all printing applications. For the remaining 25%, you must conduct a more thorough analysis of printing variables.

Note that besides the previously mentioned variables, the squeegee also has an attribute that is difficult to quantify: resistance to abrasion and chemicals. No standards or tests exist to evaluate this attribute. However, I strongly recommend that you document the performance of your squeegee materials and evaluate them on the basis of actual production. This way, at least relative to your requirements, you will be able to determine which squeegee materials perform satisfactorily. Additionally, you'll be able to determine the average useful period for particular squeegee materials with specific inks and then rotate your squeegees accordingly.

Squeegee sharpening

Since even the best squeegee edge will eventually change its shape (due to abrasion with the screen fabric), you'll need to redress the edge regularly. Typically, this is accomplished by grinding the edge on a squeegee sharpener.

When sharpening a squeegee, observe the following guidelines:

1. Use coarse grinding tools (the lower the grit, the coarser) when you have to remove a lot of material all at once.

2. In general, remove as little material as possible using the finest grit that will "cut" the material (rather than melt and smear it).

3. Make sure the squeegee edge is straight, and the blade's free height is uniform along the squeegee's entire length.

4. When printing with transparent inks or printing images that require totally uniform coverage without squeegee marks, polish the edge of the blade with a fine (400- to 600-grit) emery cloth.

5. When polishing the edge of the squeegee blade, try to bevel the edge for high-resolution images and round it for broad, uniform printed areas. A beveled edge provides a sharper printed image than a rounded one. However, it is easier to eliminate "nicks" from the squeegee edge by rounding it.

6. Always round off the ends of the squeegee on either side of the blade: Sharp ends serve no useful purpose and put undue stress on the screen mesh.

Squeegee care and storage

Getting the most out of your squeegees also means storing and maintaining them properly. Here are a few tips to help you maximize the quality and useful life of your squeegees:

1. Store squeegee blades flat, not in a roll. Long pieces should be cut to size, then stored flat.

2. Assembled squeegees (where the blade is mounted in the holder) should be stored so that the blade rests on the top end of the holder. A squeegee should never rest on the blade, and the blade should never touch anything during storage.

3. Squeegees should be cleaned immediately after use to prevent accumulation of dried-up ink. Neither the blade nor the holder should have any ink left after cleaning. (Dry ink near the edge of the blade will cause squeegee marks during printing.)

4. Squeegees should not be "left to soak" in solvents. Although they are designed to withstand most solvents, soaking will cause temporary swelling and permanent loss of resilience.

5. Unsupported squeegee blades should be rotated at least every four to six hours during production to prevent permanent deformation of the blade.

6. Squeegees age and harden with time. Do not use old blades that are 10 degrees (units of durometer) harder than their original designation. Such materials will perform erratically and will have minimum resilience and abrasion resistance.

Conclusion

As a screen printer, you have to contend with a long list of variables in order to ensure accurate color and resolution on your prints. Fortunately, the variables relating to the squeegee are among the easiest to identify and match to your job requirements. By taking care when selecting squeegees and making squeegee maintenance a regular part of your production routine, you'll be able to deliver accurate, high-quality graphics print after print.

Squeegee Terminology

Squeegee holder A wood or metal clamping device that holds the squeegee blade for manual or automatic (mechanized) printing.

Squeegee blade An elastomer blade, usually a type of polyurethane, that actually performs the printing.

b. height--the dimension perpendicular to the width, ranging from 1 to 2.5 in.

c. free-height--the free dimension of the blade that is not clamped in the holder.

d. length--the longest dimension of the blade.

Squeegee blade edge The working surface of the squeegee blade that actually makes contact with the screen during the print stroke.

Squeegee blade profile The cross-sectional shape of the squeegee blade at its printing edge.

Squeegee blade edge profile (sharpness) Edge quality based on the roundness or chamfer of the squeegee edge. The edge radius of a "sharp" squeegee is less than 0.010 in. (0.25 mm).

Squeegee blade durometer The hardness or softness of the blade material as measured by a shore A testing device. The three broad categories of blades include soft (60-70 durometer), medium (70-80 durometer), and hard (85 durometer or higher).

Multidurometer squeegee blade A squeegee blade that is made of two or three layers of different durometer elastomers. The hard elastomers prevent the bending of the blade and are typically sandwiched between layers of softer elastomers that allow adaptation to surface irregularities.

Squeegee blade stiffener or backing A metal support that runs along the entire length of the blade and prevents the blade from bending.

Set squeegee angle The angle set by the press operator between the squeegee and the screen (in the direction of the print stroke) when the squeegee is not printing.

Effective squeegee angle The actual angle between the squeegee and screen during the print stroke.